Each and every month, various games and DLC are permanently discounted on the Xbox Live Marketplace. For May 2013, a selection of Fallout 3, Oblivion, and Brink DLC gets a permanent drop, along with ilomilo and a few Games on Demand:

Brink 'Agents of Change' - 400

Fallout 3 'The Pitt' - 400*

Fallout 3 'Point Lookout' - 400*

Fallout 3 'Operation: Anchorage' - 400*

Fallout 3 'Mothership Zeta' - 400*

Fallout 3 'Broken Steel' - 400*

Oblivion 'Shivering Isles' - 800*

ilomilo - 400

Assassin's Creed III - $39.99

The Bigs 2 - $19.99

Halo 4 - $39.99

WWE All Stars - $19.99

*Prices still in the process of updating on the Marketplace and Xbox.com